2. Descriptive Astronomy (“Astronomy Without a Telescope”) http://apod.nasa.gov/apod/astropix.html • How do we locate stars in the heavens? • What stars are visible from a given location? • Where is the sun in the sky at any given time? • Where are you on the Earth? An “asterism” is two stars that appear To be close in the sky but actually aren’t In 1930 the International Astronomical Union (IAU) ruled the heavens off into 88 legal, precise constellations. (52 N, 36 S) Every star, galaxy, etc., is a member of one of these constellations. Many stars are named according to their constellation and relative brightness (Bayer 1603). Sirius α − Centauri, α-Canis declination less http://calgary.rasc.ca/constellation.htm - list than -53o not Majoris, α-Orionis visible from SC http://www.google.com/sky/ Betelgeuse https://en.wikipedia.org/wiki/List_of_Messier_objects (1758 – 1782) Biggest constellation – Hydra – the female water snake 1303 square degrees, but Ursa Major and Virgo almost as big. Hydrus – the male water snake is much smaller – 2243 square degrees Smallest is Crux – the Southern Cross – 68 square degrees Brief History Some of the current constellations can be traced back to the inhabitants of the Euphrates valley, from whom they were handed down through the Greeks and Arabs. Few pictorial records of the ancient constellation figures have survived, but in the Almagest AD 150, Ptolemy catalogued the positions of 1,022 of the brightest stars both in terms of celestial latitude and longitude, and of their places in 48 constellations. The Ptolemaic constellations left a blank area centered not on the present south pole but on a point which, because of precession, would have been the south pole c. 2800 BC, a fact that is consistent with the belief that the constellation system had its origin about 5,000 years ago. E.g., ORION Betelgeuse and Rigel are M42 = Orion nebula M43 = DeMairan’s nebula and -Orionis Winter Triangle Sirius – brightest star in the sky – star of about twice the 8.6 ly mass of the sun. Blue. Very luminous, very hot. A main sequence star (like the sun) but of Type A1 Procyon – 8th brighest star. About 1.4 solar masses. Another 11.5 ly main sequence star. Hotter and more luminous than the sun but not as luminous as Sirius. Type F5. May be close to finishing hydrogen burning as its luminosity is a bit high for its mass. Betelgeuse – 9th brightest star. 2nd brightest in Orion. 643 ly Betelgeuse is a red supergiant. It is not fusing hydrogen in its center. It has left the main sequence. May vary in brightness over periods of years by as much as a factor of two. About 18 solar masses and around 10 million years old. Winter Hexagon Orion Nebula: M-42 1600 light years away in the sword of Orion, easily visible to the naked eye. 85’ x 60’ across and part of a larger cloud spanning 20 degrees*. Diameter ~30 ly, Mass ~ 200,000 solar masses. • Your fist at arm’s length is about 10 degrees Star Nursery Betelgeuse - red supergiant, about 20 solar masses. May have shrunk 15% in radius since 1993. This probably does not indicate evolution at its center. 570 ly away. Variable star. 1000 times as luminous as the sun Rigel - brightest star in Orion by (a bit more than -Orionis = Betelgeuse – a variable) 7th brightest star in the sky. 770 ly. Most luminous star in our region of galaxy. A blue supergiant star, 17 solar masses. Brightness varies by 3 to 30% Triple star system. A is bright. B is a binary. Trapezium - an open cluster of young stars which illuminate the Orion nebula. The 5 brightest are all over 15 solar masses. Three were discovered by Galileo in 1617. optical IR *Bayer (1603) designated the brightness of 1564 stars in his Uranometria Finding the north star x 6 Your latitude is the angle above the northern horizon where you see Polaris. Polaris does not move. Motions of stars in the sky North South Polaris is 6 times the distance between the pointers away – i.e., ~30o. Can tell time this way, but a) 24 hr clock b) sidereal time A siderial day is a bit shorter than a solar day 24 hr 365.25 The apparent location of the sun in the sky as the earth goes round it defines a great circle in the heavens called the “ecliptic”. The projection of the earth’s equator in the sky gives another called the “celestial equator”. Because the Earth’s rotational axis is not perpendicular to the plane containing the earth’s orbit around the sun, the planes containing the two circles are not the same but are inclined to each other by 23.5o. The path of the sun in the sky XI/30 – XII/17 Think of the earth as being at the center of this imaginary celestial sphere 23.5o The ecliptic is fixed in the sky because the earth’s orbit doesn’t change. The celestial equator moves over 1000’s of years due to precession Where the sun is Astronomically speaking. The traditional signs of the zodiac are all 30 degrees in length and the sun spends roughly equal times in each. They are not equivalent to where the sun is. E.g., the zodiacal sign Aquarius (1/21– 2/19) roughly corresponds to Capricon below. Pisces " The Fishes " March " 12 " to "April " 18" Aries " The Ram " April " 19 " to "May " 13" Taurus " The Bull " May " 14 " to "June " 19" Gemini " The Twins " June " 20 " to "July " 20" Cancer " The Crab " July " 21 " to "August " 9" Leo " The Lion " August " 10 " to "September 15" Virgo " The Maiden " September 16 " to "October " 30" Libra " The Balance " October" 31" to "November 22" Scorpius " The Scorpion " November 23 " to "November 29" Ophiuchus** Serpent-holder " November 30 to December 17" Sagittarius The Archer " December 18 " to "January " 18" Capricornus The Goat " January" 19 " to " February" 15" Aquarius " The Water-bearer February 16 " to " March " 11" Because the planets and sun are all approximately in the same plane, the planets are also found in the constellations of the zodiac. Western astrology uses the tropical zodiac which is affixed to the vernal equinox. The siderial zodiac is fixed to the stellar background when the system started. Hindu astrology uses the siderial zodiac https://en.wikipedia.org/wiki/Zodiac - Constellations How about the apparent motion of the stars in the sky? If you stood at the earth’s north pole, your zenith would be the projection of the earth’s rotational axis into the sky. Your horizon would be the celestial equator. The stars would go round Polaris in a counter- clockwise direction The celestial equator is the projection of the Earth’s equator into the heavens. 8 hr time lapse photo The Daily Motion •! As the Earth rotates, the sky appears to us to rotate in the opposite direction. •! The sky appears to rotate around the N (or S) celestial poles. •! If you are standing at the poles, nothing rises or sets. •! If you are standing at the equator, everything rises & sets 90o to the horizon. copied from Nick Strobel’s At the equator, stars would “Astronomy notes”. See his website. all rise perpendicular to the horizon and set perpendicular to the horizon. Every day Panoramic view of the African night from equatorial Kenya. The three hour long exposure was made on a clear, dark, mid November evening facing due west and covers just over 180 degrees along the horizon. So, the South Celestial Pole is at the center of the concentric arcs on the left and the North Celestial Pole is at the far right. The stars setting along the Celestial Equator leave the straight trails near the middle of the picture. Leroy Zimmerman, Astronomy picture of the day November 15, 2002 Latitude of Seattle At a lower latitude than the north pole = 47.6 degrees 90 + 47.6 = 137.6o 180 − 90 − 47.6 = 42.4 47.6 42.4o Stars within a certain angle of the north pole would go in circles around the pole and never set. Others have more complicated paths. Some near the south pole remain invisible. Only stars on the celestial equator would rise due east and set due west. Stellar Coordinates Right Ascension and Declination •! Celestial Equator Projection of the Earth’s equator into the sky •! Declination − 90o ≤δ ≤ + 90o The angle to a star or other object in degrees, minutes, and seconds measured north or south of the Celestial Equator N CE S What can be seen from a given location (e.g. Santa Cruz)? L = 37 Celestial equator (90o away from poles) Polaris = 0 Sometimes 37o visible 90-L -53o North 37o South 37o E.g., at Santa Cruz where latitude = 37o N o Stars more than 53 above the South pole Celestial Equator will always be visible. Stars more than 53o below the CE will never be visible. The poles and the celestial equator actually latitude of SC is 36.792o` remain fixed in the sky as the earth rotates Stars will be “circumpolar”, i.e., never set if their declination is L = latitude o δ ≥ 90 − L L > 0 in the northern hemisphere and δ ≤ − 90o − L L < 0 in the southern hemisphere. Note that L is negative in the southern hemisphere. At the south pole L = -90. At the north pole L = +90. A star will rise above the horizon sometime in a 24 hour period if Northern o e.g, north pole > 0 hemisphere δ > L − 90 south pole < 0 Southern o equator 90 > > -90 hemisphere δ < L + 90 Where is the declination of the star and L is your latitude.